The present invention relates to a method for manufacturing a highly accurate optical disc master, and to a method for manufacturing an optical disc produced by using the master.
Recently, recording media that record and store a wide variety of information have been remarkably developing. In particular, regarding a compact recording medium, as the recording system is changed from a magnetic recording medium to an optical recording medium, the recording capacity has been increasing from an order of mega bytes (MB) to an order of gigabytes (GB).
The optical recording medium has changed from Compact Disc™ (CD) to an optical disc in recent years. A read-only optical disc, i.e., a digital versatile disc read-only memory (DVD-ROM), being 12 cm in diameter has an information capacity of 4.7 GB on the single side. This disc can record images that correspond to the recording for two hours in National Television System Committee (NTSC) color television system.
However, as information and communication technology and image processing technology have rapidly developed in recent years, even the above optical disc requires a several fold recording capacity relative to the present capacity. For example, a next-generation optical disc, which is an extension of a digital video disc being 12 cm in diameter, requires an information capacity of 25 GB on the single side. This disc can record images that correspond to the recording for two hours in the digital high vision system.
The optical disc is composed of an optically clear substrate, for example, polycarbonate. Fine irregular patterns such as pits and grooves that represent information signals are formed on one main surface on the substrate. A reflecting film, i.e., a metal thin film composed of, for example, aluminum, is formed on the fine irregular patterns. Furthermore, a protective film is formed on the reflecting film.
In the above recording medium, minimizing the irregular pattern can increase the recording density, and consequently, can increase the recording capacity. A process for manufacturing an optical disc, which relates to the minimizing of the irregular pattern on the optical disc, will now be described with reference to FIG. 10.
A resist layer 91 is uniformly formed on a substrate 90 (FIG. 10(a)).
Subsequently, the resist layer 91 is selectively exposed according to a signal pattern (FIG. 10(b)). The resist layer 91 is developed to produce a master 92 having a predetermined irregular pattern thereon (FIG. 10(c)). An example of the known method for producing this master will now be described.
A glass substrate having a sufficiently smooth surface is used as the substrate. The substrate is disposed on a rotatable table. While the glass substrate is rotated at a predetermined speed, a photosensitive resist, i.e., photo resist (organic resist) is applied on the substrate. The glass substrate is further rotated in order to spread the photo resist. Thus, the resist layer is formed on the whole area by spin coating. Subsequently, the photo resist is exposed with recording laser such that the photo resist has a predetermined pattern. Thus, a latent image corresponding to information signals is formed on the substrate. Then, the substrate is developed with a developer to remove the exposed areas or the unexposed areas of the photo resist. In this way, a resist master is produced. The resist master 92 includes the glass substrate and the photo resist layer formed thereon and having the predetermined irregular pattern.
Then, a metallic nickel film is formed on the irregular pattern of the resist master 92 by electroforming (FIG. 10(d)). The nickel film is lifted off from the resist master 92. Subsequently, a predetermined process is performed to produce a molding stamper 93 having the irregular pattern of the resist master 92 (FIG. 10(e)).
Polycarbonate, which is a thermoplastic resin, is molded by injection molding using the molding stamper 93 to forma resin disc substrate 94 (FIG. 10(f)). The stamper is removed (FIG. 10(g)), and then a reflecting film 95 composed of an aluminum alloy (FIG. 10(h)) and a protective film 96 are formed on the irregular surface of the resin disc substrate 94 to produce an optical disc (FIG. 10(i)).
As described above, in order to produce the fine irregular pattern on the optical disc, the pattern is reproduced on the substrate accurately and quickly by the use of the stamper on which the fine irregular pattern is formed with high precision. In terms of the precedent process, the precision of the fine irregular pattern on the optical disc depends on the cutting process, i.e., the process in which the resist layer is exposed with laser to form the latent image.
For example, according to the above read-only DVD (DVD-ROM) having the information capacity of 4.7 GB, cut portions are formed on the stamper such that a pit line (0.4 μm in the minimum pit length, 0.74 μm in the track pitch) is formed in a spiral shape. In order to form the cut portions, laser having the wavelength of 413 nm and an objective lens having the numerical aperture NA of about 0.90 (for example 0.95) are used.
The minimum pit length P (μm) to be exposed is represented by following Formula (1):P=K·λ/NA  (1)wherein λ (μm) represents a wavelength of the light source, NA represents a numerical aperture of the objective lens, and K represents a proportionality constant.
The wavelength λ of the light source and the numerical aperture NA of the objective lens depend on the specification of laser equipment, and the proportionality constant K depends on the combination of the laser equipment and the resist master.
When the optical disc having the information capacity of 4.7 GB is produced, the wavelength is 0.413 μm, the numerical aperture NA is 0.90, and the minimum pit length is 0.40 μm. Therefore, according to Formula (1), the proportionality constant K is 0.87.
On the other hand, in order to meet the demand for the optical disc having the information capacity of 25 GB, the minimum pit length must be decreased to 0.17 μm, and the track pitch must be decreased to about 0.32 μm.
In general, shortening the wavelength of the laser is effective for nanofabrication of the irregular pattern (i.e., the formation of submicron pits). As described above, in order to meet the demand for the high-density optical disc having the information capacity of 25 GB on the single side, the minimum pit length must be decreased to about 0.17 μm. In this case, if the proportionality constant K is 0.87 and the numerical aperture NA is 0.95, the light source must include laser equipment having the wavelength λ of 0.18 μm.
ArF laser having a wavelength of 193 nm has been developing so that the laser is used as a light source for semiconductor lithography for the next-generation. The above wavelength, i.e., 0.18 μm, is shorter than the wavelength of the ArF laser. An exposure system that achieves an exposure with such a short wavelength is very expensive because the exposure system requires not only the special laser used as the light source, but also special optical parts such as a special lens. Accordingly, the above method for achieving nanofabrication, in which the wavelength λ during exposure is shortened and the numerical aperture NA of the objective lens is increased in order to increase the optical resolution, is not extremely suitable for producing inexpensive devices. The reason is that, as the patterns become fine, the existing exposure systems cannot be used and more expensive exposure systems must be introduced instead. Accordingly, even if the performance of the laser equipment in an exposure system is improved, the increase of the recording capacity in the optical disc is limited.
In a general present exposing step, organic resists such as novolac resists and chemically amplified resists are exposed with ultraviolet rays as the light source. The organic resists are all-purpose and widely used in the photolithographic field. Unfortunately, the patterns on the boundaries between the exposed areas and the unexposed areas are not clear because of the high molecular weight of the organic resists. Accordingly, in terms of the precision, the organic resists cannot be used for the nanofabrication of the optical disc having a high capacity level of 25 GB.
On the other hand, inorganic resists, in particular, amorphous inorganic resists provide clear patterns on the boundaries between the exposed areas and the unexposed areas because the minimum structure unit of the inorganic resist is an atomic level. Therefore, the inorganic resists are suitable for the precise nanofabrication compared with the organic resists. The use of the inorganic resists is promising to produce the optical disc having a high capacity. For example, in a known nanofabrication process, a resist material such as MoO3 or WO3 is exposed with ion beam as the light source (see, for example, Nobuyoshi Koshida, Kazuyoshi Yoshida, Shinichi Watanuki, Masanori Komuro, and Nobufumi Atoda: “50-nm Metal Line Fabrication by Focused Ion Beam and Oxide Resists”, Jpn. J. Appl. Phys. Vol. 30 (1991) pp. 3246). In other known process, a resist material composed of SiO2 is exposed with electron beam as the light source (see, for example, Sucheta M. Gorwadkar, Toshimi Wada, Satoshi Hiraichi, Hiroshi Hiroshima, Kenichi Ishii, and Masanori Komuro: “SiO2/c-Si Bilayer Electron-Beam Resist Process for Nano-Fabrication”, Jpn. J. Appl. Phys. Vol. 35 (1996) pp. 6673). Furthermore, a process has been studied in which a resist material composed of chalcogenide glasses is exposed with laser having the wavelength of 476 nm and 532 nm, and a mercury xenon lamp that radiates ultraviolet rays as the light source (see, for example, S. A. Kostyukevych: Investigations and modeling of physical processes in inorganic resists for the use in UV and laser lithography”, SPIE Vol. 3424 (1998) pp. 20).
As described above, when ion beam or electron beam is used as the light source of the exposure, many kinds of inorganic resist material can be used in combination. In addition, the fine convergence of the electron beam or the ion beam allows the irregular patterns to be minimized. However, an apparatus having the electron beam or the ion beam as the irradiation source has a complicated structure and is very expensive. Unfortunately, this apparatus is not suitable for producing an inexpensive optical disc.
In terms of the manufacturing cost, ultraviolet rays or visible light, that is, light from, for example, laser equipment installed in the existing exposure system, is preferably used. However, a limited material of the inorganic resists can be patterned to form the cut portions using ultraviolet rays or visible light. Chalcogenide is the only material that can be patterned using ultraviolet rays or visible light so far. The materials of the inorganic resists other than chalcogenide transmit ultraviolet rays or visible light, and barely absorb the light energy. Accordingly, these materials are not suitable for the practical use.
From an economical point of view, the use of the existing exposure system and chalcogenide is a practical combination. Unfortunately, chalcogenide includes materials that are harmful to the human body, for example, Ag2S3, Ag—Ag2S3, and Ag2Se—GeSe. Therefore, in terms of the industrial production, the use of chalcogenide is difficult.
As described above, the optical disc having a high recording capacity cannot be manufactured with the existing exposure system so far.
In order to solve the above problems, it is an object of the present invention to provide a method for manufacturing an optical disc master and a method for manufacturing an optical disc having a higher recording capacity. In the method for manufacturing an optical disc master, expensive irradiation equipment having, for example, electron beam or ion beam is not used, instead, a safe resist material suitable for precise nanofabrication and the existing exposure system are used.